Aromatic amines are important intermediates which have to be prepared inexpensively and in large amounts. For this reason, production plants for aromatic amines are generally built for very high capacities. The high productivity of these plants is ensured by means of very long reaction cycles and trouble-free running between the start-up and shutdown operations of the hydrogenation for regeneration of the hydrogenation catalysts used.
Aniline is an important intermediate, e.g. for the preparation of methylenedi(phenyl isocyanate) (MDI) and is generally produced on an industrial scale by catalytic hydrogenation of nitrobenzene by means of hydrogen. Particular preference is given to reaction conditions as described in GB 1 452 466 A1, EP 0 011 090 A1 or EP 0 944 578 A2 (isothermal mode of operation) and in EP 0 696 574 B1, EP 0 696 573 B1, EP 1 882 681 A1 (adiabatic mode of operation). Apart from the abovementioned processes using stationary catalyst beds, processes using fluidized catalyst beds have also been described, e.g. in DE 1114820 B, DE 1133394 B or WO 2008/034770 A1.
In all the adiabatic and isothermal processes described, the starting material nitrobenzene is reacted with an excess of hydrogen.
The preparation of the aromatic amines is carried out in reaction cycles because the catalyst activity of the hydrogenation catalysts decreases steadily.
The activity of used catalysts for the hydrogenation of aromatic nitro compounds therefore has to be restored at periodic intervals. For this purpose, a regeneration is carried out by removing carbon-containing deposits from the catalyst by burning-off in a stream of air. In other embodiments of the process, the burning-off step is followed by a washing step, e.g. as described in U.S. Pat. No. 3,684,740. The next reaction cycle can then be started, starting up the hydrogenation plant again.
EP 0 944 578 A2 (page 2, lines 1-20) discusses the start-up procedure for an adiabatic process for the hydrogenation of nitro compounds to form the corresponding amines. There, it is stated that increasing the space velocity of aromatic nitro compounds used over the catalyst continuously or stepwise to the maximum space velocity over a period of from 10 to 1000 hours has an advantageous effect on the space-time yield.
The quality of a process for the hydrogenation of aromatic nitro compounds is firstly defined by the content of undesirable hi-products of the reaction in the product. Secondly, the quality of a hydrogenation process is defined by the entire process of hydrogenation cycle, shutdown of the hydrogenation, regeneration of the hydrogenation catalyst and start-up of the hydrogenation process being able to be operated without a technical production stoppage.
In general, supported metal catalysts which have a preferred temperature window for optimal activity are used for heterogeneously catalyzed gas-phase processes. If the maximum temperature is exceeded only briefly, damage to the catalyst, e.g. as a result of sintering processes of the active metal particles on the support material, can occur.
Although the above-described processes of the prior art make it possible to prepare aromatic amines with high selectivity and in high yield, they describe, with the exception of EP 0 944 578 A2 which describes the start-up procedure, only processes which are already in operation and are running steadily. The possible problems in the start-up of a process for preparing aromatic amines are passed over.